1. pk Field of the Invention
The present invention relates to an indirect firing fastener driving tool, including a piston mounted in a barrel for, under the action of the combustion gases of a propulsive charge, driving a fastener for fixing a work piece on a support, means for absorbing the piston being provided between the barrel and a fastener guide disposed in front of the barrel, for absorbing the excess propulsion energy of the piston and limiting its stroke, the absorbing means comprising, in combination, a ring, arranged for cooperating with the head of the piston, a first intermediate ring, made from a resilient material, disposed between the ring cooperating with the piston head and the fastener guide, and a second intermediate ring, made from a rigid but slightly deformable material, movable axially between the ring cooperating with the piston head and the fastener guide.
2. Description of the Prior Art
Fastener driving tool of this type are often used for boarding buildings, such as sheds. It is a question, for example, and using short fasteners, of fixing metal covering sheets on beams having for example an I section. Generally, the thicknesses of the support materials receiving fixing fasteners are variable from one material to another, and the regularity of driving the fasteners into these materials must be ensured. To take the example of I section beams, to be covered with metal sheets of small thickness, it frequently happens that the operators work blind, without knowing whether they are firing into the web of the beams, into their side flanges of small thickness, or even to the side. If it is in the flanges, the fasteners would risk being driven in too far, which would be prejudicial to the fixing quality, but if it is to the side, they could pass through the small thickness metal sheets in the manner of a veritable projectile propelled at a speed which is still high, if the driving tool were not in fact provided with means for absorbing the piston which limit its stroke, without mentioning tools which are based on the absorbtion of the excess power by driving in the fastener itself, and in which the piston may project out of the fastener guide.
The absorber for the piston is therefore intended to limit or control the stroke thereof. In fact, it is sufficient to ensure the position of the piston at the time when, so to speak, it releases the fastener for, at this time, the kinetic energy of the fastener only represents a very small portion of the propulsion energy, which is insufficient for it to continue its driving stroke. In other words, as soon as the fastener is released by the piston, it stops.
Several types of absorbers are already known, non elastic steel rings on the one hand and resilient material rings, for example made from polyurethane, on the other.
Steel rings offer the advantage of good driving precision, but they have the drawback of increasing the risks of breakage of the piston by sudden stopping. Resilient rings do not have this drawback. But because they are frequently driven with considerable force, they become deformed and even break up to the detriment of the positioning accuracy of the piston and the operation of the tool.
From U.S. Pat. No. 3,465,942, a driving fastener tool of the above mentioned type is known. However, in the tool of this document, the second intermediate ring acts only to avoid the first intermediate ring to flow along the piston. The assembly of these two absorbing rings of this document does not enable to determine with a very good accuracy the axial position of the piston after firings.